(thanks to K1KW for providing this info)
There is another intermittent source of RFI I found that was a bear to track down. The bottom plate of the 100D that slides out doesn't make reliable contact with the case. Add the paint that is in that groove and it's dicey contact at best. While chasing RFI, I happened to pick up the 100D, and when I pressed the bottom plate, it went away! A quick check with an ohm meter connected between the bottom plate and the case while pressing it verified this was occurring. Depending on the quality of the happenstance contact, I could change the amount of RFI. What I did to fix that was to drill and tap four 6/32 holes in the bottom of the case placed so that a 3/8 long 6/32 machine screw would clamp the bottom plate tightly. These four holes are placed over the groove that the bottom plate slides into. Fixed THAT problem.

The center shield has the same issue as the bottom plate. To fix that, I drilled a clearance hole for a 6/32 screw hole in the center shield near the rear panel and another directly opposite it in the bottom plate. I slid in a metal spacer (standoff) that was tapped on either end and screwed it down from either end. I noticed on my 100D that the center shield did seem to make contact with the sides of the case since it was a tight fit so this may be unnecessary but is insurance. It also looks like this groove was masked off so that no paint got into it.

One comment on the painted panels and removing the paint. A much easier way to get the same result is to just clean off a little paint in a circle where the screwheads contact the panel so you get metal to metal contact under the screwhead. I double down on this and also use internal tooth lock washers under the screwheads. The threads will get lots of solid contact with the case providing a solid connection. That will effectively ground the front and rear panels at enough points so that on HF it's virtually solid. Saves all that paint scraping and produces the same result.

If that bottom plate isn't grounded, it acts like an antenna, coupling any RF on it right into the 100D. Take an ohm meter and see if you get continuity between the bottom plate and the ground stud on the rear. If you do, try moving the plate or press on it and see if that changes anything.

I also scraped a small circle of paint around the holes on the rear panel where all the BNC connectors pass through. I did this on the outside where the nut and lock washers are since I could see that even the lock washers under many of the BNC nuts didn't penetrate through the thick paint!

It seems to me the plate between the PA is not adequately grounded to serve as a proper shield. I plan to bond the plate to ground using set screws to ensure a proper electrical connection. (W6IZT)